Asteroids, Comets, and Dwarf Planets: Their Nature, Orbits, and Impacts

What are asteroids like?

Asteroid Facts Asteroids are rocky leftovers of planet formation. The largest is Ceres, diameter ~1000 kilometers. 150,000 in catalogs, and probably over a million with diameter >1 kilometer. Small asteroids are more common than large asteroids. All the asteroids in the solar system wouldn’t add up to even a small terrestrial planet.

Asteroids are cratered and not round. Thought question: were these pictures taken from Earth? One of these has a satellite (Ida is orbited by Dactyl, lower left) Asteroids are cratered and not round.

Asteroids with Moons Some large asteroids have their own moon. Asteroid Ida has a tiny moon named Dactyl.

Density of Asteroids Measuring the orbit of asteroid’s moon tells us an asteroid’s mass. Mass and size tell us an asteroid’s density. Some asteroids are solid rock; others are just piles of rubble.

Asteroid Orbits Most asteroids orbit in the asteroid belt between Mars and Jupiter. Trojan asteroids follow Jupiter’s orbit. Orbits of near-Earth asteroids cross Earth’s orbit.

Origin of Asteroid Belt Rocky planetesimals between Mars and Jupiter did not accrete into a planet. Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

Where do meteorites come from?

Meteor Terminology Meteorite: a rock from space that falls through Earth’s atmosphere Meteor: the bright trail left by a meteorite

Meteorite Impact Chicago, March 26, 2003 Simple rocks and metal, occasionally carbon compounds and water. Shiny bits are metal flakes, first to condense. White features are solidified droplets of material that splashed out during impacts during accretion Chicago, March 26, 2003

Meteorites from Moon and Mars A few meteorites arrive from the Moon and Mars. Composition differs from the asteroid fragments. A cheap (but slow) way to acquire Moon rocks and Mars rocks

What are comets like?

Comet Facts Formed beyond the frost line, comets are icy counterparts to asteroids. Nucleus of comet is a “dirty snowball.” Most comets do not have tails. Most comets remain perpetually frozen in the outer solar system. Only comets that enter the inner solar system grow tails.

Sun-Grazing Comet Simple rocks and metal, occasionally carbon compounds and water. Shiny bits are metal flakes, first to condense. White features are solidified droplets of material that splashed out during impacts during accretion

Nucleus of Comet A “dirty snowball” Source of material for comet’s tail

Deep Impact Mission to study nucleus of Comet Tempel 1 Projectile hit surface on July 4, 2005. Many telescopes studied aftermath of impact.

Anatomy of a Comet A coma is the atmosphere that comes from a comet’s heated nucleus. A plasma tail is gas escaping from coma, pushed by the solar wind. A dust tail is pushed by photons.

Growth of Tail

Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.

Meteors in a meteor shower appear to emanate from the same area of sky because of Earth’s motion through space.

Where do comets come from?

Only a tiny number of comets enter the inner solar system Only a tiny number of comets enter the inner solar system. Most stay far from the Sun. Oort cloud: on random orbits extending to about 50,000 AU Kuiper belt: on orderly orbits from 30–100 AU in disk of solar system

How did they get there? Kuiper belt comets formed in the Kuiper belt: flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction

Icy Dwarf Planets

Pluto’s Orbit Pluto will never hit Neptune, even though their orbits cross, because of their 3:2 orbital resonance. Neptune orbits three times during the time Pluto orbits twice.

Is Pluto a Planet? Much smaller than the terrestrial or jovian planets Not a gas giant like other outer planets Has an icy composition like a comet Has a very elliptical, inclined orbit Has more in common with comets than with the eight major planets

Other Icy Bodies There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune. The largest of these, Eris, was discovered in summer 2005, and is even larger than Pluto.

Kuiper Belt Objects These large, icy objects have orbits similar to the smaller objects in the Kuiper belt that become short period comets. So are they very large comets or very small planets?

What are the large objects of the Kuiper belt like?

What is Pluto like? Its moon Charon is nearly as large as Pluto itself (probably made by a major impact). Pluto is very cold (40 K). Pluto has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto’s orbit takes it farther from the Sun.

Hubble’s View of Pluto and Its Moons

Other Kuiper Belt Objects Most have been discovered very recently so little is known about them. NASA’s New Horizons mission will study Pluto and a few other Kuiper belt object in a planned flyby.

Are Pluto and Eris planets?

Pluto and Eris Pluto’s size was overestimated after its discovery in 1930, and nothing of similar size was discovered for several decades. Now other large objects have been discovered in Kuiper belt, including Eris. The International Astronomical Union (IAU) now classifies Pluto and Eris as dwarf planets. Dwarf planets have not cleared most other objects from their orbital paths.

Have we ever witnessed a major impact?

Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter.

An impact plume from a fragment of comet SL9 rises high above Jupiter’s surface.

Dusty debris at an impact site

Artist’s conception of SL9 impact

Several impact sites

Impact sites in infrared light

Did an impact kill the dinosaurs?

Likely Impact Site Geologists have found a large subsurface crater about 65 million years old in Mexico.

Is the impact threat a real danger or media hype?

Facts about Impacts Asteroids and comets have hit Earth. A major impact is only a matter of time: not IF but WHEN. Major impacts are very rare. Extinction level events happen millions of years apart. Major damage happen tens to hundreds of years apart.

Tunguska, Siberia: June 30, 1908 Several atomic bombs worth of energy. Tunguska, Siberia: June 30, 1908 A ~40-meter object disintegrated and exploded in the atmosphere.

Meteor Crater, Arizona: 50,000 years ago (50-meter object) Crater is 1 km in diameter. Impact was 20 megatons Privately owned National Landmark. Meteor Crater, Arizona: 50,000 years ago (50-meter object)

Frequency of Impacts Small impacts happen almost daily. Impacts large enough to cause mass extinctions happen many millions of years apart.

The asteroid with our name on it We haven’t seen it yet. Deflection is more probable with years of advance warning. Control is critical: Breaking a big asteroid into a bunch of little asteroids is unlikely to help. We get less advance warning of a killer comet….